In order to increase the power reserve, by increasing the number of turns of a mainspring, one solution consists in decreasing the diameter of the barrel arbour and of the associated core, so as to increase the space available for the spring inside the drum.
The ratio of the core radius to the spring thickness is usually comprised between 10 and 20 and the invention proposes to reduce this ratio to below 10, and preferably to within a range of between 5 and 10.
The sizing must not be too small; there is a risk of breakage if the core diameter is too small.
In the conventional barrel architecture, a ratchet is axially mounted on a barrel arbour or on a core, via a square, with the ratchet usually being secured by an axial screw. The dimension of this screw and that of the square thus define the minimum diameter of a pivot shoulder. A step portion joined to this pivot shoulder limits the endshake of the arbour or of the core relative to a bottom plate or to a bridge carrying a jewel or similar element.
An even larger diameter than that of the step portion is required for a shoulder for guiding the pivoting of the drum on the arbour or on the core, combined with a step portion limiting the endshake of the drum. The dimension chain required to observe minimal sections of material results in substantial dimensions, which are difficult to reduce. In particular, it is not sufficient merely to reduce all of the dimensions, since the cross-sections of material are then insufficient to ensure fatigue resistance.
U.S. Pat. No. 996,499 A in the name of MARSHALL discloses a barrel wherein the ratchet is integral with a single-piece cover which includes the hook for hooking the outer coil of the balance spring. The cover is guided by a shoulder in the plate, and includes a bore in which the end of the barrel arbour is guided. The barrel arbour has a main cylindrical shoulder, which guides the drum via two bearings. This shoulder is limited by a stop shoulder, which holds the drum at a distance from the bridge, in which one end of the barrel arbour rotates. Whereas the back cover includes a step portion arranged to limit its travel relative to the plate. The mainspring is thus not mounted between the arbour and drum, but between the drum and the cover.
CH Patent Application No 318898A in the name of JUNGHANS discloses a barrel which includes a drum having a pipe on which the core pivots; the core is connected, via a square, to the barrel arbour pivoting between the bridge and plate, and is clamped to the ratchet. On the opposite side to the ratchet, the arbour carries a toothed wheel cooperating with the self winding mechanism. A fillet on a bridge limits the clearance of this toothed wheel.
U.S. Pat. No. 1,033,020A in the name of KERN discloses a barrel with a cover for closing the drum fixed to the drum by a bayonet and pin, the cover carrying the toothing.
EP Patent Application No 1542098A2 in the name of LANGE concerns a particular configuration wherein the drum is guided by its periphery in a bore in the bridge, and wherein the drum includes a projecting annular surface, called a support area, which extends radially beyond the guide bore in the drum so as to provide a support, or stop member, for the drum on the bridge.